A method of the general class to which the invention relates is described in European Patent application No. 0084383.
In known two-stage grinding methods the quantity of grit recirculated in the grinding system fluctuates somewhat as a function of the grindability of the material. In order to maintain optimum crushing conditions in the second mill, which for example may be a ball mill, it is known for the quantity of fresh material to be regulated as a function of the quantity of recirculated grit so that the sum of the quantity of fresh material and the quantity of grit, and thus the degree of filling of the second mill, remains constant.
If the speed of a roll mill were to be maintained constant, alteration of the quantity of fresh material supplied therto would result in a change in the material filling level in the delivery chute of the roll mill. This would entail an alteration in the grinding conditions in the roll mill and in, certain circumstances, would lead to the delivery chute running full or running empty.
In order to avoid this disadvantage it is known to vary the speed of the roll mill to be altered appropriately when the quantity of fresh material is altered (i.e., adapted to the quantity of grit at that time) in order in this way to achieve a constant filling level in the delivery chute of the roll mill. However, such a method requires the use of comparatively expensive variable-speed drives for the two rolls of the roll mill.
The object of the invention, therefore, is to provide comparatively low expense apparatus and a method which ensure optimum crushing conditions.